Jeff Atwood has an interesting post on Vista's memory management and how it differs from Windows XP. Vista treats your system memory as a cache and uses it aggressively. In other words, don't be surprised or upset if Vista consumes all your available memory. It's not a bug, it's a feature!
In summary, here's how much faster each cache memory type in your computer is than the hard drive:
System memory
37x faster
CPU Level 2 cache
82x faster
CPU Level 1 cache
283x faster
Those figures explain why I only have 6 megabytes of "free" memory in Windows Vista. Vista is trying its darndest to pre-emptively populate every byte of system memory with what it thinks I might need next. It's running a low-priority background task that harvests previously accessed data from the disk and plops it into unused system memory. They even have a fancy marketing name for it-- SuperFetch: